1. Field of the Invention
This invention relates generally to semiconductor devices and more specifically to a high power semiconductor laser diode with an integrated taper that provides, on a reproducibility basis, for a substantially circular mode profile for efficient coupling thereof to an optical fiber.
2. Background of the Invention
The efficient coupling of light to optical fibers is a critical aspect of many optical communication systems and industries. In particular, low fiber coupling efficiency of laser diodes has been a major limitation for high power single mode fiber output. In a conventional laser diode, optical confinement in the semiconductor structure is asymmetric and the propagating mode profile is elliptical in shape. Also, the mode profile of these high power diode laser sources is reflected in laser beam divergence. The stronger divergence is normally in the vertical or transverse direction due to the strong optical confinement in the vertical direction in the layered semiconductor laser structure, as opposed to the weaker optical confinement in the horizontal or lateral direction.
This highly divergent, elliptical laser diode output beam profile presents a difficulty when attempting to couple the light from a high power laser diode source to a single mode optical fiber. This difficulty is primarily due to the large mode mismatch between the semiconductor laser source and the optical fiber. Thus, a high power laser diode with a circular mode profile and a narrow far-field divergence is particularly desirable for efficient fiber coupling.
Semiconductor waveguide devices have been developed to try to solve this mode-matching problem. These devices have been designed to be either stand-alone passive waveguides or passive waveguides integrated into an active optical source. In particular, several conventional waveguide devices utilize a tapered waveguide region. Briefly, a taper region in a semiconductor waveguide device acts to adiabatically control the expansion of propagating wave and, therefore, the resultant mode-size of the guided optical wave. These tapers may be formed through various techniques. Typically, waveguide devices with lateral tapers are lithographically produced using sub-micron masking and etching procedures as exemplified in U.S. Pat. No. 5,574,742 to Ben-Michael et al. However, it is difficult to precisely control these lithographic processes and the resultant tapers are not easily reproducible. Vertical tapers can be more desirably fabricated using one of several different other techniques. These techniques include selective area growth/etch; shadow mask growth/etch; dip etch; and multiple etch-stop layers. These techniques are briefly described in the article of T. Brenner et al. entitled, "Integrated Optical Modeshape Adapters in InGaAsP/InP for Efficient Fiber-to-Waveguide Coupling," IEEE Photonics Technology Letters, Vol. 5, No. 9, 1053-1056 (September, 1993) with reference to other articles or disclosures where the technique is described in particular detail. However, those techniques require extremely high degree of reproducibility in growth etch in order to control the beam divergence as desired. Also, many of the devices fabricated by the conventional techniques in the past suffer from a high degree of optical loss during adiabatic mode conversion. Thus, the light emitted from these conventional tapered waveguide devices can have improved output beam mode quality in terms of divergence, but at the cost of lower optical output power.
What would be more desirable is a simpler and more reproducible structure with highly functional monotonically diminishing vertical taper that provides good expansion characteristic and low optical loss to the propagating beam from the active section to the passive section of the device.
The simplest and easiest method to fabricate a vertical taper so far is the one developed and demonstrated by T. Brenner et al. in the articles entitled, "Vertically Tapered InGaAsP/InP Waveguides for Highly Efficient Coupling to Flat-End Single-Mode Fibers", Applied Physics Letters, Vol. 65(7), pp. 798-800, Aug. 15, 1994 and "Compact InGaAsP/InP Laser Diodes With Integrated Mode Expander for Efficient Coupling to flat-Ended Single Mode Fibres", ELECTRONICS LETTERS, Vol. 31(17), pp. 1443-1445, Aug. 17, 1995, both of which are incorporated herein by their reference. Here, short in tracavity mode expanders employ concurrent formation of a vertical taper, formed by diffusion-limited etching, and a lithographically formed horizontal waveguide taper to accommodate expanded optical mode. However, the techniques of T. Brenner et al. require high reproducibility in growth/etch rate in order to control the beam divergence in a reproducible manner. Further, a significant difficulty with the proposed structure is that the vertical taper continuously extends so that in order to achieve the desired mode profile, the cleaving of the front end facet position becomes critical relative to the exact point of cleave. Because the dimensions of these devices are in the micron range, it is difficult on a continuous reproducibility basis to form a cleave in a place where the thickness of the waveguide layer will be virtually the same in each case. This is because the reproducibility of the same identical taper from wafer to wafer is difficult to achieve. Therefore, there is no guarantee that the desired mode profile will be exactly the same each time or sufficiently optimized to be substantially circular in extent. Thus, what is need is some built-in mechanism to insure that the mode profile is reproducible by providing a "tolerance window" within which the front facet can be made, resulting in every case the achievement of substantially the same desired mode profile.
Therefore, it is an object of this invention to provide a method of forming an improved tapered waveguide structure that performs adiabatic mode conversion of the elliptical diode laser output into a substantially reproducible circular mode profile.
A further object of this invention is to provide an easier method for fabricating vertical and horizontal waveguide tapers in semiconductor laser devices that provides high reproducibility of the desired mode profile in the output beam from the device.